In the prior art, like SAAB's PCT/SE91/818 and Toyota's U.S. Pat. No. 7,047,917, a pair of connecting shafts is arranged at the two sides of the cylinder block, laterally, to connect the upper and lower sections of the engine. The rotation of a control shaft displaces the cylinder head relative to the crankcase to vary the compression ratio. The inevitable long distance between the two connecting shafts generates heavy bending loads, flexing and noise, making the reinforcement of the two sections inevitable.